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I was looking for a little input from you guys/gals on a new frame I am considering purchasing. Based on my current bike which fits well, I seem to be in between sizes. I can see pros/cons to either size. Based on the following comparative parameters, which do you think is the better fit? FYI I currently have an effective top tube of 548mm and a HT of 155mm with 20mm top cone/spacer and a -6d / 110mm stem. Also, I only ride for fun and do weekend rides of about 30mi and several local rallies throughout the season of about 50+ miles. In other words, purely non competitive amateur.

53 pros: Seems to match my current frame pretty well minus the shorter HT. 53 cons: More spacers? yikes.55 pros: Potentially could run minimal spacers (aesthetic +). Taller HT might be better as I age with the bike (turning 35). 57 cons: I would need a shorter stem or different bars to reduce reach. A 100mm would probably do the trick.

This is only my second frame so the only baseline I have is my first and current bike. I haven't switched frames before so I have no way of knowing if these differences listed are negligible or not. Also, unfortunately, assume I won't be able to take either for a test ride before hand.

Nice plots! You've done your homework, which is really nice. A lot of people post similar questions without even thinking about the most basic issues.

I assume you matched BB position, in which case it appears BB drop is perfectly matched between the two. Matching BB is best.

It looks to me like the head tube is steeper on the 55 than the 53. Is this true? This may make for snappier handling with a reduced trail. Other than that, my personal preference is to err on the smaller side since it's lighter, but that's just my bias. I don't worry about slamming the stem aesthetic.

I often say to people that two sizes of the same bike will usually fit the same rider, assuming 2cm incraments. But they fit in different ways. If you were a teenager or early twenties and into racing, I would maybe say 53. But being a mid-thirties recreational rider ... 55cm. A stack of headset spacers do not necessarily say that your bike is a poor fit, but they sure look crap.

Thanks for the input. Yes I took the plots I made in bikecad.ca 's software into photoshop, gave them a color, then aligned them on the BB. Good eye! The headtube is indeed steeper on the 55, 73.5d vs 73d on the 53 (which would have more trail).

I am so on the fence. Part of me is like you and wants to go with the smaller size. However, if I don't have to, I'd rather not end up with 35mm - 40mm of spacers to get a fit similar to my current bike. Decisions...decisions...

I wouldn't either especially since 40mm of spacers is the limit on most forks, and on some it is less.So flip the stem up. It's slightly lighter and stiffer that way anyhow. With a 6 degree stem that will cut roughly 20mm off the spacers.

I've had my Cervelo R3s (2006 and 2009) that way since 2006 and I have yet to hear a comment in the real world about it. Not that I would care.

It seems to only be in forums that people think stem position is a significant indicator of riding ability.

I think as long as you can get the position you want without using more than 2cm of spacers on the HT and without using crazy stem lengths, frame size is largely irrelevant. I just went through this whole decision myself--I could have either a 54 or 52 cm Caad10. I chose the 52 since I prefer a very low bar 120mm -17 stem. The key thing to remember is that you put the seat in the same position relative to the BB (fore-aft and height) on whatever bike you end up with. In my example, the seat would have been a bit farther forward on the rails to get it in the right spot on the 54 as compared to the 52. I would not have been able to get my low bar height with the 54, though, reach could easily have been the same. Obviously you're looking for a much higher bar (I am a Cat. 2 racer, but am 37), so the larger frame may allow you to get that without resorting to spacers. The whole tiny frame/long slammed stem/saddle back on the rails thing is driven by people like me who ride 300+ miles a week and are looking for maximum performance, comfort be damned (though I did ride my race bike for over 6 hours last Sunday with no complaints). A much more sensible fit would have most of us on larger frames.

I say go with the 55, out of your examples. If you find yourself becoming more flexible and aggressive, you can always use a -17 stem.

Thanks to all that have replied. I think I've settled on the 55. Overall 1cm of reach difference (between my current frame and the 55 frame) is not drastic. I can easily accommodate that with a stem change or bar change (with shorter reach). I'd rather roll with a HT that is similar to what I have now with a minimal stack versus a more aggressive/shorter HT given that I am not a racer or looking for that kind of fit as you all have pointed out.

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